Jack Dempsey was a famous American boxer around the 1920's. When this fish species from Guatemala/Belize was first introduced in the USA, I guess some smart importer was trying to find a cool name to make them sell better, and chose that one as an allusion to their aggressiveness.

It's pretty much the same idea behind the commercial naming of other American cichlids like Red Devil, Green Terror, etc...it's mostly just a marketing pitch..._________________Success with a fish/tank is measured in YEARS, not months or weeks...